1. Field of the Invention
The present invention relates to an analog filter circuit incorporated in a semiconductor integrated circuit and, more particularly, to an improvement of a filter adjusting circuit.
2. Description of the Related Art
FIG. 1 shows a conventional analog filter circuit. In FIG. 1, reference numeral 20 denotes a filter adjusting circuit; and 21, a filter circuit.
The filter circuit 21 is constituted by amplifying npn transistors Q15 and Q22, the collectors of which are connected to a VCC potential, npn transistors Q16 and Q23, the emitters of which are commonly connected and collectors and bases are connected to each other, a resistor R8 connected between the emitter of the transistor Q15 and the collector of the transistor Q16, a resistor R12 connected between the emitter of the transistor Q22 and the collector of the transistor Q23, an npn transistor Q18 for current source and a resistor R9 connected in series between a common emitter connecting point of the transistors Q16 and Q23 and a ground potential GND, an npn transistor Q17, the base of which is connected to the base of the transistor Q16, and the collector of which is connected to the VCC potential, an npn transistor Q20, the emitter of which is connected to the emitter of the transistor Q17 and the base of which is connected to the base of the transistor Q23, a resistor R10 and a pnp transistor Q19 connected in series between the VCC potential and the collector of the transistor Q20, a capacitor C connected between the collector of the transistor Q19 (a signal output node 22) and the VCC potential, and an npn transistor Q21 for current source and a resistor R11 connected in series between a common emitter connecting point of the transistors Q17 and Q20 and the ground potential GND.
An analog input voltage VIN is applied to the base of the amplifying transistor Q15. The base of the transistor Q22 is connected to the collector of the transistor Q19. An output voltage VOUT is output from the output node 22 to which the collector of the transistor Q19 is connected.
On the other hand, the filter adjusting circuit 20 is constituted by a constant current source I0 connected in series between the VCC potential and the ground potential GND, an npn transistor Q0, the collector and base of which are connected to each other, and a resistor R0, an npn transistor Q1, the base of which is connected to the base of the transistor Q0, a resistor R1 connected between the emitter of the transistor Q1 and the ground potential GND, a pnp transistor Q2 which is connected between the VCC potential and the collector of the transistor Q1, and the base and emitter of which are connected to each other, a pnp transistor Q3, the base of which is connected to the base of the transistor Q2, and the emitter of which is connected to the VCC potential, a resistor R2 connected between the collector of the transistor Q3 and the ground potential GND, an npn transistor Q8, the base of which is connected to the collector of the transistor Q3, an npn transistor Q10, the emitter of which is connected to the emitter of the transistor Q8, a resistor R4 connected between a common emitter connecting point of the transistors Q8 and Q10 and the ground potential GND, a pnp transistor Q7 which is connected between the VCC potential and the collector of the transistor Q8, and the base and emitter of which are connected to each other, a pnp transistor Q9, the base of which is connected to the base of the transistor Q7, the emitter of which is connected to the VCC potential, and the collector of which is connected to the collector of the transistor Q10, an npn transistor Q12, the base of which is connected to the collector of the transistor Q9, and the emitter of which is connected to the base of the transistor Q10, a resistor R5 and a pnp transistor Q11 connected in series between the VCC potential and the collector of the transistor Q12, a pnp transistor Q13, the base of which is connected to the transistor Q11, a resistor R6 connected between the VCC potential and the emitter of the transistor Q13, an npn transistor Q14, the collector and base of which are connected to each other and a resistor R7, the npn transistor Q14 and the resistor R7 being connected in series between the collector of the transistor Q13 and the ground potential GND, and a variable resistor VR which is connected outside the integrated circuit between the base of the transistor Q10 and the external ground potential for the integrated circuit, and which has an extremely low temperature coefficient.
The transistors Q8 and Q10 constituting a differential pair form a buffer amplifier circuit. The transistors Q0 and Q1 form a current mirror circuit CM1. The transistors Q2 and Q3 form a current mirror circuit CM2. The transistors Q7 and Q9 form a current mirror circuit CM3 to be a load of the buffer amplifier circuit. The transistors Q11 and Q13 form a current mirror circuit CM4. The transistors Q11 and Q13 forming the current mirror circuit CM4 have emitter areas twice those of other transistors, respectively.
The bases of the transistors Q0 and Q14 in the filter adjusting circuit 20 are respectively connected to the bases of the transistors Q18 and Q21 in the filter circuit 21. Resistances of the resistors R2, R8 and R12 are set so as to satisfy relation R2=R8=R12. The base of the transistor Q11 in the filter adjusting circuit 20 is connected to the base of the transistor Q19 in the filter circuit 21. Resistances of the resistors R5, R6, and R10 are set so as to satisfy relation R5=R6=R10/2.
Note that, temperature coefficients of the resistors R0 to R2 and R4 to R12 in the integrated circuit are equal to each other.
In the filter adjusting circuit 20, the base potential of the transistor Q0 of the current mirror circuit CM1 receiving a current from the constant current source I0 is supplied to the base of the transistor Q18 in the filter circuit 21. A current from the constant current source I0 is supplied to the resistor R2 through the current mirror circuits CM1 and CM2, so that a potential difference appears across both terminals of the resistor R2. The voltage across the resistor R2 is supplied to both terminals of the variable resistor VR outside the integrated circuit by the buffer amplifier circuit. A current proportional to a current supplied to the variable resistor VR is supplied to the transistor Q14 and the resistor R7 through the current mirror circuit CM4. The base potential of the transistor Q14 is supplied to the base of the transistor Q21 in the filter circuit 21.
In the filter circuit 21, a potential difference proportional to the first potential difference generated across the resistor R2 in the filter adjusting circuit 20 appears across each of the resistors R8 and R12. Characteristics of the filter circuit 21 are controlled by a current supplied to the transistor Q21 receiving a base current from the filter adjusting circuit 20. A transfer function of the filter circuit 21 can be obtained by the following equation. ##EQU1## where, ##EQU2##
re0 is the A.C. emitter equivalent resistance of Q18,
IC18 is a collector current of the transistor Q18, ##EQU3##
re1 is the A.C. emitter equivalent resistance of Q21,
IC21 is a collector current of the transistor Q21, ##EQU4##
k is a Boltzmann's constant,
q is an electric charge of an electron, and
T is an absolute temperature.
In addition, a time constant T0 of the filter circuit 21 is given by: ##EQU5## In addition, since equation (6) can be established, equation (7) can be obtained. ##EQU6## Since equation (7) includes a term V.sub.T, T0 changes with a change in temperature.
That is, the filter circuit 21 constitutes a low-pass filter, so that a ratio between the currents IC18 and IC21 of the filter circuit 21 is adjusted by the adjustment of the variable resistor VR connected outside the integrated circuit through the filter adjusting circuit 20, so that the current IC18 is kept constant. Therefore, by adjusting a magnitude of the current IC21, an error of the time constant T0 caused by the fluctuation of the capacitor C of the filter circuit 21 can be adjusted so that the time constant T0 can be kept to be a predetermined value at a predetermined temperature.
In the conventional filter circuit, however, the characteristics of the transistors of the filter circuit 21 are changed by the change in temperature, so that the time constant is changed.
As described above, the conventional analog filter circuit has a problem that characteristics of transistors of a filter circuit are changed by a change in temperature, so that a time constant is changed.